In the production of magnetic disks and disk drives for computers, an important step in the production process is the testing of the disks to locate defects thereof. Due to high volume of production, it is extremely important to quickly and reliably locate defects which may occur in the manufacture of the disk drives.
A typical magnetic disk or a platter has recording surfaces on both sides, and conventionally a plurality of such disks are included in a head disk assembly (HDA). For example, personal computers now commonly use HDAs. A read/write head is positioned near each surface, and several read/write heads are governed by a read/write control chip.
Typically, one head is associated with each surface of a given disk, although there may be more than one head coupled with a disk surface or side in certain apparatus. The disks in a head disk assembly are generally configured into a parallel disk stack. Typically, the disk stack is constantly spinning, and each time a read/write control chip causes a head to write on a given track on one side of a disk, the disk stack rotates once.
Governed by control signals external to the head disk assembly, each chip causes each head which it controls to write data onto, and read data from, a given track of the associated surface of the magnetic disk. In the standard testing process, the heads are accessed in one at a time by the read/write control chips. Each track is written during one revolution of the disks. Since each head is selected one at a time, the total number of revolutions required to write a complete cylinder can be determined. A cylinder is the group of tracks of all the disks which are accessed by the heads when the heads are at a given radial position. The heads are moved together radially inwardly and outwardly to access different cylinders.
The number of revolutions for writing a given cylinder in conventional apparatus is equal to the number of heads. In order to test the entire HDA, the write process must first be carried out with test data for each track, and the number of revolutions required is the number of heads times the number of tracks. Each time test data is written onto a track of a disk, it is then read out by the same read/write head, and again the heads are accessed for this purpose in a serial fashion by the read/write control chips. The data which is read out is analyzed for determining whether errors were introduced into the data by the write/read process, attributable to defects on the disks.
Since the read/write heads are accessed in a serial fashion both for the write function and for the read function, the total number of rotations required for conventional analysis is 2 times 12N or 24N rotations, in the example of a 6-disk stack with 12 sides for testing and N tracks per side. In general, the total number of rotations or revolutions required for a given stack will be 2.times.S.times.D.times.N, where D is the number of disks in the stack and S is the number of sides per disk to be tested.